It is known to polymerize acrylic monomers in suspension; by this process a system is carried out wherein an acrylic monomer is generally suspended under the form of drops in a continuous phase and polymerized by utilizing an initiator of radical type soluble in the monomer. The continuous phase is generally water. The ratio between continuous phase (water) and discontinuous phase (monomer) is generally comprised between 1:1 and 3:1. The final product is formed by a suspension of polymer particles (beads) with a diameter of 0.1-1 mm easily removable from water by centrifugation.
In the practical embodiment of this type of process, it is necessary the employment of suspending stabilizers hindering the coalescence of the small drops of monomer in the most advanced steps of the polymerization.
As suspending stabilizers hydrosoluble macromolecular compounds with affinity towards the monomer are used, which, placing themselves at the interface between organic phase and aqueous phase, form a protective film hindering agglomeration of the particles. At the end of the polymerization the suspending agent is removed from the surface of the polymer particles by washing with water.
The suspending agent is a key factor as its characteristics condition the performances of the whole process both from the point of view of the quality of the final product and from the point of view of manufacture costs, depending on the specific consumptions of raw materials, productivity, etc.
Various polymerization processes in aqueous suspension of acrylic monomers in general are known, among which the one described in European patent application 457,356, wherein particular polymers selected from the homopolymers of compounds of formula ##STR2## are used as suspending and stabilizing agents of the aqueous suspension, wherein: R.sub.1 =H, CH.sub.3 ; R.sub.2 and R.sub.3, equal or different, are H, alkyls C.sub.1 -C.sub.8, linear or branched when possible and M=alkaline or alkaline-earth metal, or copolymers of said compounds with acrylic monomers.
Tests carried out by the Applicant have shown that the polymerization process in suspension with the suspending agents indicated in the European patent application mentioned above in the water/monomers ratios comprised between 1:1 and 3:1 is unable to produce the copolymers object of the present invention.
The Applicant from experiments carried out has verified that when monomers soluble in water are copolymerized the stability of the suspension is compromised and agglomeration phenomena occur which can, in the most drastic cases, involve the whole polymeric mass and block the stirring and, in the less dramatic cases, produce formation of crusts and polymer blocks on the walls and on the buffles of the polymerization reactors. In any case these phenomena make the process not industrializable unless the reaction criticality is reduced by operating for instance with high water/monomer ratios and/or with rather slow polymerization kinetics. In any case the modifications above mentioned will damage the productivity.
Other organic or inorganic suspending agents are well known in the art, for instance polyvinylic alcohol, hydroxyalkylcellulose, etc.; among the inorganic ones for instance tricalcic phosphate in combination with the so called extenders (surfactants).
However the drawback of these suspending agents resides in that the polymers obtained show optical properties not suitable to the typical fields of the acrylic products for instance, lighting, car lights, signes, etc.
Polymerizations in suspension in non aqueous mediums (solvents) are also known, however there are here drawbacks due to the higher costs of solvent and polymer recovery and safety problems due to inflammability and explosiveness of solvents.
Other systems known in the art to produce the copolymers of the invention can be in solution: the acrylic monomers are dissolved in a suitable solvent and then polymerized by radical way. The copolymers of the invention can be produced with this method, however there is the drawback of the removal of the solvent from the polymer as said above.
Alternative processes to those in solution are those in mass, wherein the solvent is the monomer itself.
Both these processes, in solution and in mass are generally utilized in continuous from the industrial point of view to obtain higher productivities, the volumes of equipments utilized being the same, compared with a semicontinuous process. The drawback of continuous processes resides in that they result not very flexible from the industrial point of view.
Generally, the manufacture volumes of copolymers with hydrosoluble comonomers up till now do not justify a continuous dedicated plant. In this case a continuous process would require frequent campaign changes with big recovery problems of the unreacted monomers and of the polymer produced during the campaign transition.
The copolymers of the invention can be produced also in emulsion, however there are higher plant and processing costs owing to the separation of the emulsion. Besides the obtained copolymer results optically less pure for the presence of surfactants and/or coagulants.
It was felt the need to have at one's disposal an industrial process allowing to produce copolymers containing the comonomers soluble in water with high yields combined with good mechanical and optical properties typical so the high quality acrylic products and obtained with multipurpose (discontinuous) plants.
It has now been unexpectedly and surprisingly found that this is possible by using the process described hereunder.